Impaired dilation of coronary arterioles during increases in myocardial O₂ consumption with hyperglycemia

Previous studies showed that nitric oxide (NO) plays an important role in coronary arteriolar dilation to increases in myocardial oxygen consumption (MVO₂). We sought to evaluate coronary microvascular responses to endothelium-dependent and to endothelium-independent vasodilators in an in vivo model. Microvascular diameters were measured using intravital microscopy in 10 normal (N) and 9 hyperglycemic (HG; 1 wk alloxan, 60 mg/kg iv) dogs during suffusion of acetylcholine (1, 10, and 100 μM) or nitroprusside (1, 10, and 100 μM) to test the effects on endothelium-dependent and -independent dilation. During administration of acetylcholine, coronary arteriolar dilation was impaired in HG, but was normal during administration of nitroprusside. To examine a physiologically important vasomotor response, 10 N and 7 HG control, 5 HG and 5 N during superoxide dismutase (SOD), and 5 HG and 4 N after SQ29,548 (SQ; thromboxane A₂/prostaglandin H₂ receptor antagonist) dogs were studied at three levels of MVO₂: at rest, during dobutamine (DOB; 10 μg · kg^-1 · min^-1 iv), and during DOB with rapid atrial pacing (RAP; 280 ± 10 beats/min). During dobutamine, coronary arterioles dilated similarly in all groups, and the increase in MVO₂ was similar among the groups. However, during the greater metabolic stimulus (DOB+RAP), coronary arterioles in N dilated (36 ± 4% change from diameter at rest) significantly more than HG (16 ± 3%, P < 0.05). In HG+SQ and in HG+SOD, coronary arterioles dilated similarly to N, and greater than HG (P < 0.05). MVO₂ during DOB+RAP was similar among groups. Normal dogs treated with SOD and SQ29,548 were not different from untreated N dogs. Thus, in HG dogs, dilation of coronary arterioles is selectively impaired in response to administration of the endothelium-dependent vasodilator acetylcholine and during increases in MVO₂.